Programmable signal emphasis for optical transmitters

ABSTRACT

A method includes delivering an electrical signal over an electrical conductor to a transmitter module, which includes a memory holding compensation values corresponding respectively to different conductor lengths. One or more of the compensation values is retrieved from the memory of the transmitter module, and a waveform of the electrical signal is modified responsively to the retrieved compensation values. The electrical signal having the modified waveform is transmitted using the transmitter module over a communication link.

FIELD OF THE INVENTION

The present invention relates generally to communication systems, andparticularly to methods and systems for pre-emphasizing signalstransmitted over optical links.

BACKGROUND OF THE INVENTION

Various communication networks communicate over optical links. Forexample, IEEE 802.3ae-2002™ standard, Aug. 30, 2002, and IEEE DraftP802.3aq™/D4.0, May, 2006, which are incorporated herein by reference,define physical layer specifications for communicating over opticalfibers. Section 52-5 of the IEEE 802.3ae-2002 standard specifies bothShort Range (SR) and Long Range (LR) fiber links. Section 68.5 of theIEEE P802.3aq™/D4.0 draft defines a Long Range Multimode (LRM) fiberlink.

Optical fiber links are typically terminated with optical transceivers,which convert optical signals to electrical signals and vice versa.Optical transceivers may conform to various specifications and formfactors, such as, for example, the XENPAK, X2, XFP, Small Form-factorPluggable (SFP) and Small Form-factor Pluggable Plus (SFP+) formfactors. The SFP+ form factor, for example, is specified in aspecification published by the SFP+ committee, entitled “Specificationsfor Enhanced Small Form Factor Pluggable Module,” (SFF-8431), revision3.2, Nov. 12, 2008, which is incorporated herein by reference. The XFPform factor is specified in “10 Gigabit Small Form Factor PluggableModule,” (INF-8077i), revision 4.0, the SFP+ committee, Apr. 13, 2004,which is incorporated herein by reference. The Xenpak form factor isspecified in “A Cooperation Agreement for 10 Gigabit EthernetTransceiver Package,” Issue 3.0, Sep. 18, 2002, which is incorporatedherein by reference. The X2 form factor is specified in “A CooperationAgreement for a Small Versatile 10 Gigabit Transceiver Package,” Issue2.0b, Apr. 7, 2005, which is incorporated herein by reference.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are block diagrams that schematically illustrate anoptical line card, in accordance with an embodiment of the presentinvention;

FIG. 3 is a graph showing a pre-emphasized electrical signal, inaccordance with an embodiment of the present invention;

FIG. 4 is a flow chart that schematically illustrates a method forpre-emphasizing an electrical signal, in accordance with an embodimentof the present invention; and

FIG. 5 is a block diagram that schematically illustrates a setup forcalibrating signal emphasis parameters, in accordance with an embodimentof the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

In various types of optical communication equipment, an electricalsignal traverses an electrical conductor (e.g., a circuit trace) beforeit is converted into an optical signal. For example, line cards inoptical switches typically deliver electrical signals over circuittraces to optical transmitters. The optical transmitters convert theelectrical signals into optical signals for transmission over opticalfibers.

In many practical cases, the circuit trace distorts the electricalsignal traveling through it. Typically, the circuit trace attenuateshigh-frequency components of the electrical signal. This sort ofdistortion degrades the quality of the transmitted optical signalproduced by the optical transmitter, and sometimes prevents the opticalsignal from meeting its specified quality level.

The distortion caused by the circuit trace can be corrected bypre-emphasizing the electrical signal, so as to enhance itshigh-frequency components and compensate for the frequency-selectivecircuit trace attenuation. In some cases, however, the distortion of theelectrical signal depends on the length of the circuit trace, as well ason the properties of the specific optical transmitter to which theelectrical signal is provided. Therefore, the optimal amount ofpre-emphasis that should be applied to the electrical signal alsodepends on these factors.

Embodiments of the present invention that are described here in belowprovide improved methods and systems for pre-emphasizing electricalsignals in optical communication equipment. When pre-emphasizing a givenelectrical signal that traverses a given circuit trace en-route to agiven optical transmitter, the disclosed methods and systems take intoaccount the actual trace length in question and the properties of thespecific optical transmitter.

In some embodiments, the optical transmitter is integrated with amemory, to form an optical transmitter module. The optical transmittermodule conforms to a certain form factor, such as SFP+. The memory holdsmultiple compensation values, each compensation value corresponding to acertain circuit trace length. Each compensation value in the memoryspecifies the amount of pre-emphasis that, if applied to the electricalsignal, would best compensate for the distortion caused by a circuittrace having the corresponding length.

Pre-emphasis of the electrical signal is applied by a compensation unit,such as an Electric Dispersion Compensator (EDC), which is controlled bya processor. In order to pre-emphasize the electrical signal, theprocessor reads one or more of the compensation values from the memoryof the corresponding optical transmitter module. The processor computesthe appropriate compensation value based on the values read from thememory and on the actual length of the circuit trace. The actual circuittrace length is assumed to be known (e.g., pre-programmed) to theprocessor in advance. The processor then configures the correspondingEDC to apply the computed compensation value to the electrical signal.

Since the memory is an integral part of the optical transmitter module,the compensation values stored in the memory can account formodule-specific properties, e.g., internal wiring or connectors in thetransmitter module. Such properties may differ, for example, from onetype of module to another (e.g., between modules produced by differentvendors), between individual modules, and/or from one lot of modules toanother.

In some embodiments, the compensation values stored in the memory of theoptical transmitter module are determined by a-priori characterizationof the module and the EDC for different circuit trace lengths. Anexample of a calibration setup for performing such a characterization isdescribed herein.

In summary, the disclosed methods and systems enable accuratepre-emphasis of an electrical signal, by accounting for the actualcircuit trace length and for the specific properties of the opticaltransmitter module involved. The accurate electrical signal pre-emphasisimproves the quality of the optical signal, and thus the overallperformance of the optical communication equipment.

System Description

FIG. 1 is a block diagram that schematically illustrates an optical linecard 20, in accordance with an embodiment of the present invention. Card20 produces one or more electrical signals and converts them intorespective optical signals, in order to transmit the optical signalsover optical fiber links. For example, card 20 may be part of an opticalswitch or other communication equipment.

Card 20 comprises one or more Serializer/Deserializer (SERDES) devices24, which produce the electrical signals. In a typical line cardconfiguration, the line card is connected to a motherboard (not shown inthe figures), and the SERDES devices produce the electrical signals inresponse to data that is received from the motherboard. Alternatively,the electrical signals can be produced by any other suitable signalsource on card 20.

The electrical signals produced by SERDES devices 24 are delivered overelectrical conductors, such as circuit traces 28, and are provided tooptical transmitter modules 32. The electrical signals typicallycomprise train pulses having rates in the range of 1.25 to 10.3125 Gb/s.Alternatively, any other suitable pulse rates can be used. Each opticaltransmitter module converts the input electrical signal into arespective optical signal. The optical signal is then transmitted overan optical fiber (not shown in the figures) that is connected to themodule.

(The methods and systems described herein are mainly concerned withoptical transmission rather than reception. Therefore, modules 32 arereferred to herein as optical transmitter modules. Typically, however,modules 32 comprise optical transceivers, which perform opticaltransmission as well as reception.)

Card 20 typically comprises a Printed Circuit Board (PCB) on whichtraces 28 are disposed. Card 20 and traces 28 may be fabricated usingdifferent materials and processes. For example, card 20 may comprise asingle-layer of multi-layer PCB. The PCB may be fabricated from anysuitable substrate material, such as Flame Retardant 4 (FR4). Traces 28may comprise differential micro strip traces, strip line traces or anyother suitable type of electrical conductors.

Modules 32 may conform to different specifications and form factors,such as XENPAK, X2, XFP, SFP or SFP+. In the embodiments describedherein, modules 32 conform to the SFP+ form factor, and are thereforesometimes referred to as SFP+ modules. The methods and systems describedherein, however, are not limited to this particular form factor, and maybe used with optical transceiver modules having any other suitable formfactor.

The optical signal at the output of module 32 is often expected to meeta certain quality level. The quality level may be defined in terms ofvarious properties of the optical signal, such as the signal jitter, theeye mask of the signal, the signal Transmitter Waveform DispersionPenalty (TWDP), the signal extinction ratio, or any other suitableproperty. For example, the IEEE 802.3ae-2002 standard, cited above,defines a test point denoted TP2 at which the optical signal quality isto be evaluated. Table 52-7 of this standard, for example, specifiestransmitter requirements for 10 GBASE-SR links.

The optical signal, however, is often subject to various kinds ofimpairments, which distort its quality level. The distortion may becaused by both optical and electrical elements. In particular, anydistortion of the electrical signal that is provided to module 32 willappear in the optical signal.

In many practical cases, circuit traces 28, which deliver the electricalsignals to SFP+ module 32, distort the electrical signals passingthrough them. Typically, the circuit traces have a frequency responsethat declines over frequency. In other words, the circuit traces tend toattenuate higher-frequency components of the electrical signal more thanlower-frequency components. The amount of high-frequency attenuationtypically increases with the length of the circuit trace, meaning thatlong traces cause higher distortion than short traces. As a result ofthis effect, an electrical signal traversing a given circuit trace willtypically suffer from frequency-selective attenuation, whose severitydepends on the length of the trace.

In order to compensate for distortion in the electrical signals, card 20comprises compensation units, such as Electrical Dispersion Compensators(EDCs) 36. Each EDC is able to modify the waveform of an incomingelectrical signal, so that the electrical signal at its output will havea reduced distortion level. Typically, the EDC applies a digitalequalization operation to the electrical signal, using one or morefilter coefficients (taps) that can be programmed externally. EDCcomponents that can be used for this purpose are produced, for example,by Applied Micro Circuits Corporation (AMCC—Sunnyvale, Calif.), NetlogicMicrosystems (formerly Aeluros, Mountain View, Calif.), Broadcom Corp.(Irvine, Calif.), Cortina Systems, Inc. (Sunnyvale, Calif.) and VitesseSemiconductor Corp. (Camarillo, Calif.).

In the context of signal distortion, the length of a given circuit traceis typically measured from the point the electrical signal is produced,regenerated, re-timed, re-sampled or otherwise manipulated to the pointthe signal is provided to the SFP+ module. For example, if noregeneration or re-timing is performed between the SERDES and the SFP+module, the trace length is measured between the SERDES output and theSFP+ module input. In the embodiments described herein, however, thecompensation mainly concerns the stretch of circuit trace between EDC 36and SFP+ module 32. Typically but not necessarily, the distortionintroduced by the circuit trace between the SERDES and the EDC iscorrected by applying fixed compensation by the EDC.

Line card 20 further comprises a processor 40, which controls andconfigures the different elements of line card 20. In particular,processor 40 configures EDCs 36 using data that is read from SFP+modules 32, using methods that are described in detail below. Thefunctions of processor 40 may be implemented in software, using hardwareor firmware elements, or using any suitable combination of hardware,firmware and software elements. In some embodiments, processor 40comprises a general-purpose processor, which is programmed in softwareto carry out the functions described herein. The software may bedownloaded to processor 40 in electronic form, over a network, forexample, or it may, alternatively or additionally, be provided and/orstored on tangible media, such as magnetic, optical, or electronicmemory.

FIG. 2 is a block diagram that schematically illustrates line card 20,and in particular some internal structure of SFP+ module 32 and EDC 36,in accordance with an embodiment of the present invention. EDC 36comprises a programmable pre-emphasis filter 44, which filters theelectrical signal traversing circuit trace 28. An example of apre-emphasis operation is shown in FIG. 3 below. Filter 44 may comprisedigital and/or analog circuitry, and may apply linear and/or non-linearfiltering, as appropriate. Filter 44 is configurable by processor 40. Inother words, processor 40 can program filter 44 to apply the appropriatepre-emphasis operation to the electrical signal.

In some embodiments, in addition to filter 44, EDC 36 comprises anamplifier (not shown in the figure) whose gain can be controlled byprocessor 40. In these embodiments, processor 40 may set the desiredcompensation value by controlling both the gain of the EDC amplifier andthe filter taps. This technique increases the achievable granularity ofsetting the compensation value. For example, in a typical EDC, settingthe amplifier gain to one of three possible levels provides agranularity of 1-1.5%, in comparison to ˜4% granularity achieved in asingle gain setting of the amplifier.

(Since the present patent application is mainly concerned with signaltransmission, elements of EDC 36 that are related to signal receptionwere omitted from the figure for the sake of clarity. In practice,however, EDC 36 often performs dispersion compensation to the receivedsignals, as well.)

Module 32 comprises an optical transducer 50, which converts theelectrical signal into an optical signal at the desired wavelength andinterfaces with the optical fiber. In addition, module 32 comprises aninternal memory 48, such as an Electrically Erasable Programmable ReadOnly Memory (EEPROM), which can be accessed by processor 40. Opticaltransducer 50, memory 48, and any other circuitry of module 32, aretypically packaged in a mechanical package that conforms to the SFP+form factor. Typically, module 32 is detachable and can be inserted intoand removed from card 20, as desired.

Compensation for Electrical Signal Distortion Depending on Trace Lengthand SFP+ Module Properties

The internal structure and configuration of SFP+ module 32 may affectthe actual trace length that distorts the electrical signal. Suchvariations may comprise, for example, effects caused by internal wiringor connectors in the module. Thus, variations from one module 32 toanother may cause the actual trace length to vary, as well. Thesevariations may comprise unit-to-unit variation or lot-to-lot variationscaused, for example, by manufacturing tolerances. Moreover, module 32may have different versions that conform to the same interfacespecification and form factor but differ in internal design. Forexample, different versions of module 32 may be produced by differentvendors.

A practical implication of these variations is that the appropriatecompensation, which should be applied by EDC 36, may vary depending onthe specific SFP+ module 32 to which the signal is provided.

In order to account for these variations when applying distortioncompensation by EDC 36, memory 48 of SFP+ module 32 holds a table 52 ofpre-programmed compensation values, which specify the compensation thatshould be applied to the electrical signal provided to this SFP+ module,for various circuit trace lengths. Typically, a set of lengths isdefined, so as to cover the overall range of possible trace lengths. Forexample, table 52 may be pre-programmed with compensation valuescorresponding to trace lengths between one and eight inches. The tracelengths in the table need not necessarily be distributed uniformly overthe range. For example, the table may hold compensation values for anumber of trace lengths that have been pre-measured in a calibratedsetup. An example of such a setup is shown in FIG. 5 below.

Since memory 48 is an integral part of module 32, the compensationvalues programmed into table 48 may take into account effects that arespecific to the particular SFP+ module. An example of a process fordetermining the appropriate compensation values for a given module 32 isdescribed further below.

In order to compensate for the distortion caused to a given electricalsignal by a given circuit trace 28 and a given SFP+ module 32, processor40 reads one or more of the compensation values from memory 48 of theSFP+ module. The actual length of the circuit trace in question isknown, and is assumed to be provided (e.g., pre-programmed) to processor40 in advance. Processor 40 configures EDC 36 on this circuit tracebased on (1) the known actual trace length and (2) the compensationvalues read from memory 48 of the SFP+ module. An example of a processof this sort is described in FIG. 4 below.

Processor 40 may read memory 48 and configure EDC 36 in response to acertain triggering condition, such as when the line card is reset,switched on or connected to power, upon detecting that an SFP+ modulehas been inserted to the card, or in response to any other suitablecondition or event.

FIG. 3 is a graph showing a pre-emphasized electrical signal, inaccordance with an embodiment of the present invention. A curve 56 showsthe electrical signal at the output of EDC 36, in response to a steppulse input. In the present example, the input signal comprises asequence of sixty-four “0” pulses, followed by sixty-four “1” pulses. Ascan be seen in the figure, the EDC pre-emphasizes the leading edge ofthe electrical signal. The leading edge of the signal reaches a maximumvoltage denoted V_(PEAK,) and then stabilizes at a nominal voltagedenoted V_(TOP). The amount of pre-emphasis (also referred to as acompensation value) is defined as (V_(PEAK)−V_(TOP))/V_(TOP).

The pre-emphasis operation applied by EDC 36 enhances the high-frequencycomponents of the signal, and therefore compensates for thehigh-frequency attenuation caused by the circuit trace. The amount ofpre-emphasis is configured by processor 40, using the methods describedherein. Although the example of FIG. 3 refers to pre-emphasis of theleading edge of the electrical signal, pre-emphasis can also be appliedto the trailing edge of the signal, or to both leading and trailingedges.

Compensation Method Description

FIG. 4 is a flow chart that schematically illustrates a method forpre-emphasizing an electrical signal, in accordance with an embodimentof the present invention. The description that follows refers to asingle SFP+ module 32 and a single EDC 36 that are connected to aparticular circuit trace 28. Typically, however, the method is carriedout by processor 40 on line cards that contain multiple SFP+ modules,EDCs and circuit traces.

The method begins when processor 40 detects that EDC 36 should beconfigured. As noted above, the processor may detect that an SFP+ modulehas been inserted to a port connected to the EDC in question.Additionally or alternatively, the method of FIG. 4 can be initiated atpower-on or after reset of card 20. It is also assumed that the actuallength of the circuit trace in question is known to processor 40 inadvance. In some embodiments, the EDC is initially configured with acertain default compensation value, which does not take into account theeffects of the specific SFP+ module.

Processor 40 accesses memory 48 of SFP+ module 32 and reads one or moreof the compensation values stored in table 52, at a memory access step60. In some embodiments, processor 40 reads all the compensation valuesin the table. Alternatively, the processor may read only some of thevalues, e.g., values that are close to the actual length of trace 28.

Processor 40 identifies a compensation value in table 52, whosecorresponding trace length is nearest to the known actual length oftrace 28, at a nearest length identification step 64. For example,consider the following example of table 52:

Data Data size Dec Hex address (bytes) Description value value 224 1Comp value for case 1 26% 0x1A (1.6 inches) 225 1 Comp value for case 230% 0x1E   (2 inches) 226 1 Comp value for case 3 34% 0x22   (4 inches)227 1 Comp value for case 4 37% 0x25   (5 inches) 228 1 Comp value forcase 5 38% 0x26   (6 inches) 229 1 Comp value for case 6 40% 0x28   (8inches) 230 1 Comp value for case 7 . . . . . . 231 1 Comp value forcase 8 . . . . . . 232 1 Comp value for case 9 . . . . . . 233 1 Compvalue for case 10 . . . . . . 234 1 Reserved 235 1 Reserved 236 1Reserved 237 1 Reserved 238 1 Reserved 239 1 Reserved 240 1 Reserved 2411 Reserved 242 1 Reserved 243 1 Reserved 244 1 Reserved 245 1 Checksumfor fields 224-244

The “Data address” column gives the memory addresses in memory 48. The“Data size” column gives the data size of each data item in the table.Each entry comprises the compensation value that should be applied byEDC 36 for the given trace length, in both decimal and hexadecimalrepresentation. The decimal value indicates the value of(V_(PEAK)−V_(TOP))/V_(TOP), as defined in FIG. 3 above. The hexadecimalvalue provides the value that is to be written by processor 40 to EDC 36in order to achieve this value. The rows of the table are associatedwith “cases,” having respective trace lengths. Typically, a best fittrend line having a certain slope and offset is found for this set oftrace lengths, such as using testing and/or simulation. This process isdescribed further below. In some embodiments, different trend lines arecomputed for different types of traces, stacked connectors, active vs.passive traces, different PCB materials or configurations, etc. In thepresent example, however, the first six cases correspond to FR4 circuittraces having six different lengths.

Consider, for example, a scenario in which the actual trace length is3.5 inches. As can be seen in the table, the nearest table entry has atrace length of 4 inches. Processor 40 therefore reads from the table acompensation value of 34%, which corresponds to a trace length of 4inches.

Processor 40 then corrects the identified nearest table entry to accountfor the difference between the trace length of the nearest table entryand the actual length of trace 28, at a length correction step 68. Inthe example above, the difference is 4−3.5=0.5 inches.

In some embodiments, a best fit calculation is performed on the knowntable entries (i.e., the compensation values and corresponding tracelengths). The best fit calculation provides the marginal contribution ofevery additional inch of circuit trace to the desired compensationvalue. In the present example, fitting the six known compensation valuesto a straight line provides a slope of 0.036. In other words, everyadditional inch of circuit trace length adds 3.6% to the desiredcompensation value. Thus, in the present example, the correctedcompensation value is 34−(4−3.5)*3.6=32.2%.

In some cases, the dependence of compensation value on trace length maynot be linear. Thus, processor 40 may apply the correction of step 68using other means, such as by applying polynomial fitting to the tableentries.

In some embodiments, processor 40 further corrects the desiredcompensation value in order to remove offsets that are present in theentries of table 52, at an offset removal step 72. These offsetsoriginate, for example, from inaccuracies in the calibration setup inwhich the table entries are obtained. In these embodiments, processor 40holds one or more offset correction tables, which contain offsetcorrections for different compensation values.

In some embodiments, two offset tables are used. One table, denoted“post-emphasis correction offset,” holds fixed offsets related to thedifferent ports of the optical equipment in question. The table holdsthe fixed offsets together with the corresponding trace lengths. Asecond table, denoted “corrected emphasis” holds the compensation valueto be applied after the application of the methods described herein.

Processor 40 looks-up the offset correction table with the compensationvalue computed at step 68 (32.2% in the present example), and retrievesthe appropriate offset correction factor corresponding to thiscompensation value. In the present example, a compensation value of32.2% corresponds, after offset correction, to a corrected value of29.38%.

Having corrected the offset, processor 40 configures EDC 36 to apply theappropriate pre-emphasis compensation value (29.38% in the presentexample), at a compensation configuration step 76. This compensationvalue is sometimes referred to as an actual compensation value. Fromthis point, the EDC is configured to apply pre-emphasis that accountsfor the specific SFP+ module and the actual length of the circuit trace.As a result, EDC 36 is able to correct the distortion caused by thecircuit trace with high accuracy, so as to maintain the desired qualitylevel of the optical signal at the output of SFP+ module 32.

Calibration Setup for Measuring Compensation Values

In some embodiments, the compensation values that are pre-programmedinto memory 48 of module 32 are measured using a dedicated calibrationsetup. The calibration process described below is typically performed onSFP+ modules of different versions (e.g., produced by differentvendors).

FIG. 5 is a block diagram that schematically illustrates a setup 80 forcalibrating signal compensation values, in accordance with an embodimentof the present invention. Setup 80 comprises a reference line card 84,on which several circuit traces 28 having different lengths aredisposed. Each circuit trace is connected to a port into which an SFP+module 32 can be inserted. The opposite side of each trace is terminatedby a test port 86.

In the example of FIG. 5, card 84 comprises two traces 28. In practice,however, the setup may comprise any desired number of circuit traces.For example, the six compensation values in the table addressed in thedescription of FIG. 3 above may be measured using a reference line cardhaving six traces of different lengths.

In order to measure the compensation value for a given trace on card 84,a SFP+ module is inserted into the port in question. The respective testport 86 is connected to a test board 88, which comprises an EDC.

The calibration setup further comprises a signal source 92, in thepresent example a laser that is modulated by a Programmable PulseGenerator (PPG). Signal source 92 generates an optical signal thatemulates the rate and waveform of the optical signals produced by linecard 20. The optical signal is input to the SFP+ module, which convertsthe optical signal to an electrical signal. The electrical signaltraverses trace 28, exits card 84 at test port 86 and enters EDC 36 ontest board 88.

The EDC pre-emphasizes the electrical signal in accordance with thecompensation value it is configured to apply, and outputs the signalback to test port 86. The pre-emphasized signal traverses trace 28again, and is converted to an optical signal by SFP+ module 32. Theoutput optical signal is monitored by a digital oscilloscope 96. Theoscilloscope displays the signal, and sometimes measures one or morefigures-of-merit that are indicative of the optical signal quality. Forexample, the oscilloscope may display an eye mask of the optical signal,and may measure the signal TWDP, jitter, Data-Dependent Jitter (DDJ),Optical Modulation Amplitude (OMA), Data Dependent Pulse Width Shrinkage(DDPWS), Voltage Modulation Amplitude (VMA), or any other suitablefigure-of-merit that is suitable for evaluating the quality of opticalsignals. Additionally or alternatively, the quality of the opticalsignal can be measured or evaluated manually by a user using theoscilloscope display.

For example, the compensation value can be optimized for IEEE TP2compliance, for maximizing the eye mask margin (e.g., in SR and LRlinks), for joint eye mask maximization and TWDP minimization (e.g., inLRM links), or to meet any other quality-related criterion.

The optimal compensation value of EDC 36 for the given trace length isfound by configuring the EDC with different compensation values. Whenthe EDC comprises a configurable-gain amplifier as well as aconfigurable filter, setting the given compensation values may compriseconfiguring both the amplifier and the filter, so as to achieve highergranularity.

The optical signal quality is monitored by the oscilloscope and/or bythe user for each compensation value. The compensation value, whichcauses the optical signal monitored by oscilloscope 96 to have the bestquality, is output as the value to be stored in memory 48 of SFP+ module32. (Reference line card 84 and the circuit traces disposed thereon areassumed to be of the same type, materials and technology as those ofline card 20, in which the calibration results are to be used.Typically, the process is repeated for each type of EDC.)

In some embodiments, calibration measurements are carried out at severalinput impedances to card 84 (e.g., impedances between 90 and 110 Ohm).The measurements at the different impedances can be averaged.Alternatively, the set of measurements acquired at impedance that isclosest to the actual impedance of line card 20 can be selected.

In some cases, the compensation values measured using calibration setup80 contain offsets that are caused, for example, by differences betweenthe calibration setup and the line card in which the SFP+ modules andEDCs will be deployed. Such offsets can be estimated by and stored in anoffset table in processor 40, as explained above.

For example, offsets that are caused by test board 88 (e.g., by thecircuit traces or connectors of the test board) can be measured by (1)connecting the test board output directly to oscilloscope 96 and (2)finding the optimum compensation values of the EDC with card 84bypassed. As another example, the offset caused by the external testboard and the cables and connectors that connect this board to card 84can be measured by (1) performing a similar calibration measurement onan active circuit trace of an actual line card 20 having the same lengthas one of the traces of card 84, and (2) comparing the optimumcompensation values obtained on the calibration setup and on the actualline card.

Although the embodiments described herein mainly address electricalsignal pre-emphasis in optical transmission equipment in order tocompensate for distortion caused by circuit traces, the principles ofthe present invention can also be used for other applications, such asfor transmission over direct attach copper cable links. Example linksare defined in the SFF-8431 standard, cited above. Compensation forcircuit trace distortion in these applications can increase theachievable copper cable length.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. A method, comprising: delivering anelectrical signal over an electrical conductor to a transmitter module,which includes a memory holding a plurality of compensation values eachcorresponding to a different conductor length, wherein the compensationvalues each include a portion to compensate for a distortion of theelectrical signal caused by the transmitter module; retrieving one ormore of the compensation values from the memory of the transmittermodule; modifying a waveform of the electrical signal responsively tothe retrieved compensation values; and transmitting the electricalsignal having the modified waveform using the transmitter module over acommunication link.
 2. The method according to claim 1, wherein thetransmitter module comprises an optical transmitter module, and whereintransmitting the electrical signal comprises converting the electricalsignal having the modified waveform into an optical signal.
 3. Themethod according to claim 2, wherein the optical transmitter moduleconforms to a Small Form-factor Pluggable Plus (SFP+) form factor. 4.The method according to claim 2, and comprising pre-calculating thecompensation values held in the memory by monitoring a quality level ofthe optical signal and storing in the memory the compensation valuesthat optimize the quality level at the respective, different conductorlengths.
 5. The method according to claim 2, wherein modifying thewaveform comprises determining an actual compensation value to beapplied to the waveform, and configuring an Electrical DispersionCompensation (EDC) device to apply the actual compensation value.
 6. Themethod according to claim 5, wherein configuring the EDC devicecomprises jointly setting a filter tap and an amplifier gain of the EDCdevice.
 7. The method according to claim 1, wherein the electricalconductor comprises a circuit trace disposed on a Printed Circuit Board(PCB).
 8. The method according to claim 1, wherein the electricalconductor has a given length, and wherein modifying the waveformcomprises computing an actual compensation value, to be applied to thewaveform, responsively to the retrieved compensation values and to thegiven length of the electrical conductor.
 9. The method according toclaim 8, wherein computing the actual compensation value comprisesselecting one of the retrieved compensation values, which corresponds toa conductor length that is nearest to the given length, and correctingthe selected compensation value responsively to a difference between theconductor length of the selected compensation value and the given lengthof the electrical conductor.
 10. The method according to claim 1, andcomprising pre-calculating the compensation values held in the memoryusing a calibration setup, wherein modifying the waveform comprisescorrecting an offset in the retrieved compensation values, which iscaused by the calibration setup.
 11. The method according to claim 1,wherein the electrical signal comprises a sequence of electrical pulseshaving respective rising edges and trailing edges, and wherein modifyingthe waveform comprises emphasizing at least one edge type of the pulsesselected from a group of types consisting of the rising edges and thefalling edges.
 12. The method according to claim 1, wherein transmittingthe electrical signal comprises sending the electrical signal having themodified waveform over a direct attached copper link.
 13. The method ofclaim 1, wherein the electrical conductor has an effective length thatis increased by internal wiring and/or connectors in the transmittermodule, wherein the portion of the compensation value to compensate forthe distortion of the electrical signal caused by the transmitter moduleis configured to compensate for an effective length increase caused bythe internal wiring and/or connectors, and wherein retrieving the one ormore of the compensation values comprises: retrieving a compensationvalue that corresponds to the effective length of the electricalconductor that includes the internal wiring and/or connectors in thetransmitter module.
 14. An apparatus, comprising: an optical transmittermodule, comprising: an optical transducer operative to convert anelectrical signal, which is delivered thereto over an electricalconductor, into an optical signal, and a memory, which is configured tohold a plurality of compensation values each corresponding to adifferent conductor length and wherein the compensation values eachinclude a portion to compensate for a distortion of the electricalsignal caused by the optical transmitter module; a compensation unit,which is coupled to modify a waveform of the electrical signal providedto the optical transmitter module responsively to one or moreexternally-configurable parameters; and a processor, which is coupled toretrieve one or more of the compensation values from the memory of theoptical transmitter, to calculate the parameters responsively to theretrieved compensation values, and to configure the compensation unitwith the calculated parameters.
 15. The apparatus according to claim 14,wherein the optical transmitter module conforms to a Small Form-factorPluggable Plus (SFP+) form factor.
 16. The apparatus according to claim14, wherein the electrical conductor comprises a circuit trace disposedon a Printed Circuit Board (PCB).
 17. The apparatus according to claim14, wherein the electrical conductor has a given length, and wherein theprocessor is coupled to compute an actual compensation value, to beapplied to the waveform, responsively to the retrieved compensationvalues and to the given length of the electrical conductor, and toconfigure the compensation unit to apply the actual compensation value.18. The apparatus according to claim 17, wherein the processor iscoupled to select one of the retrieved compensation values, whichcorresponds to a conductor length that is nearest to the given length,and to correct the selected compensation value responsively to adifference between the conductor length of the selected compensationvalue and the given length of the electrical conductor.
 19. Theapparatus according to claim 14, wherein the compensation values held inthe memory are pre-calculated using a calibration setup, and wherein theprocessor is coupled to correct an offset in the retrieved compensationvalues, which is caused by the calibration setup.
 20. The apparatusaccording to claim 14, wherein the compensation values held in thememory optimize a quality level of the optical signal at the respective,different conductor lengths.
 21. The apparatus according to claim 14,wherein the compensation unit comprises a filter having at least oneconfigurable tap and an amplifier having an adjustable gain, and whereinthe processor is coupled to configure the compensation unit with thecalculated parameters by jointly setting the configurable tap and theadjustable gain.
 22. The apparatus according to claim 14, wherein theelectrical signal comprises a sequence of electrical pulses havingrespective rising edges and trailing edges, and wherein the compensationunit is coupled to modify the waveform by emphasizing at least one edgetype of the pulses selected from a group of types consisting of therising edges and the falling edges.
 23. The apparatus of claim 14,wherein the optical transmitter module comprises internal wiring and/orconnectors that increase an effective length of the electricalconductor, and wherein the portion of the compensation value tocompensate for the distortion of the electrical signal caused by thetransmitter module is configured to compensate for the effective lengthincrease caused by the internal wiring and/or connectors in thetransmitter module.
 24. An apparatus, comprising: a package whichsubstantially conforms to a form factor specified by an opticaltransceiver standard; an optical transducer disposed in the packageconfigured to receive an electrical signal via an electrical conductorand to convert the electrical signal into an optical signal; and amemory disposed in the package and configured to hold compensationvalues for modifying a waveform of the electrical signal responsively toa length of the electrical conductor and wherein the compensation valueseach include a portion to compensate for a distortion of the electricalsignal caused by internal wiring and/or connectors in the package. 25.The apparatus according to claim 24, wherein the package conforms to aSmall Form-factor Pluggable Plus (SFP+) form factor.